1. Field of the Invention
The present invention is related to a modulation device, a modulation method, and a recording medium for preventing the copy of digital information signals that are recorded on an optical disc, a digital magnetic tape and the like recording medium.
2. Description of the Related Art
With the advent of the digital multimedia age, large amounts of digital information signals are recorded on optical discs, digital magnetic tapes and the like.
For example, a read-only optical disc, such as CD (Compact Disc) for recording music information, CD-ROM (CD-Read Only Memory) for recording computer data, comprises a disc substrate in the form of a platter in which spiral or concentric tracks are defined. A variety of digital information signals can be recorded on the tracks and quickly accessed from the tracks, while the discs are suited to mass-production at a low price, and therefore, such discs have been heavily used.
On the other hand, digital magnetic tapes for recording PCM music information are heavily utilized since these tapes can be played back for a longer time than optical discs.
Meanwhile, the recording medium illustrated in the following explanation as a medium for recording digital information signals is an optical disc on which digital information signals are recorded or from which digital information signals are reproduced by the use of an optical pickup. However, digital magnetic tapes are substantially different from optical discs only in that a magnetic head is used for reproducing digital information signals, and therefore the explanation of the case making use of a digital magnetic tape is dispensed with.
The optical disc such as CD, CD-ROM as described above is formed by converting digital information signals into a digital bit sequence in the form of concave pits and convex lands, engraving the bit sequence on spiral or concentric recording tracks of a stamper disc, placing the stamper disc formed with the recording tracks in an injection molding device, transferring the recording surface of the stamper disc having the recording tracks onto a transparent disc substrate in the form of a platter made of a transparent resin having an outer diameter of 120 mm or 80 mm, a central bore diameter of 15 mm and a thickness of 1.2 mm, and further forming a reflective film and a protection film, in this order, on the recording surface to provide a read only disc.
Then, during the readout operation from a read-only type optical disc, a laser beam for reading data is emitted from an optical pickup movably provided in an optical disc drive, projected onto the recording surface through the transparent optical disc substrate on the recording surface, reflected by the reflective film of the recording surface, and received by the optical disc drive for reproducing the signals of the recording surface carried by the laser beam as reflected.
Incidentally, the music information recorded on a CD and the computer data recorded on a CD-ROM are protected by a copyright law. However, since the information and the data are digital information, it is possible to copy the information in a CD-R (Compact Disc-Recordable) which can be written to once, CD-RW (Compact Disc-ReWritable) which can be written and rewritten several times and the like writable type optical disc.
While having the similar external appearance as a read-only type optical disc such as a CD, a CD-ROM or the like, a writable type optical disc such as a CD-R, a CD-RW is formed by forming concave grooves in the form of a spiral or concentric circles on a transparent optical disc, spin-coating an organic dye to form a recording layer on the concave groove, and further forming a reflective film and a protection film, in this order, on the organic dye, and is available at a low price on the market.
In this situation, when the music information recorded on a CD or the computer data recorded on a CD-ROM is recorded on a CD-R or a CD-RW, the signal format of the CD-R or the CD-RW is the same as the signal format of the CD or the CD-ROM, and therefore the infringement of a copyright occurs.
In what follows, the code word sequence recorded on a known CD will be explained.
FIG. 1 is a view for explaining the format of EFM signals of the music information recorded on a CD. FIG. 2 is a table for use in an 8–14 modulation. FIGS. 3A and 3B are views for explaining a DSV control scheme applied after the 8–14 modulation.
First, music information is recorded on a CD in a signal format in conformity with the CD format “Red Book standards”, or the IEC (International Electrotechnical Commission) 908 standard.
In this case, generally speaking, the pit lengths of an optical disc have to satisfy a minimum run-length (a shortest pit length or a shortest land length) required under the physical restriction relating to the pit generation and the optical transmission characteristics for read/write operations, a maximum run-length (a longest pit length or a longest land length) required in order to facilitate clock regeneration, and further have to satisfy the modulation scheme requirement that the DC component and the low frequency components of recording signals as modulated are sufficiently suppressed for the purpose of securing the servo bandwidth and so forth.
The EFM (Eight to Fourteen Modulation: 8–14 modulation) scheme used for CDs is one of the modulation schemes conforming to the above restrictions and has a minimum run-length of 3T (T is the length of a channel bit) and a maximum run-length of 11T.
Namely, music original data AD to be recorded on a CD is digital data composed of a plurality of successive data units each of which comprises upper eight bits (one byte) plus lower eight bits (one byte) totaling to 16 bits (two bytes).
When the music original data AD is recorded on a glass master disc for mastering, the music original data AD is converted into a signal sequence suitable for recording in a signal format according to the EFM scheme. The EFM signals 1 are recorded on the glass master disc in the signal sequence as shown in FIG. 1. Then, a metallic master disc, a mother disc and a stamper disc are formed from the glass master disc in this order by electroforming, followed by placing the stamper disc in an injection molding device and transferring the recording surface of the stamper disc to a transparent optical disc to obtain a CD. Accordingly, the recording surface of the CD is equivalent to the recording surface of the glass master disc.
In this case, in accordance with the format of the EFM signals 1 as described above, the EFM signals 1 are prepared in the form of first and second code word sequences 1d and 1f as shown in FIG. 2 by dividing the input music original data AD into an input data word D of the upper eight bits and an input data word D of the lower eight bits, converting each of the respective input data words D of p bits (=eight bits) into run-length limited code of q bits (=eleven bits) (referred to as a code word C in the following description) while conforming to the run-length limiting rule of the minimum run-length of 3T and the maximum run-length of 11T with reference to a coding table as shown in FIG. 2, and concatenating each adjacent code words C converted as shown in FIG. 1 with an intervening merge bit sequence 1b of r bits (=three bits) for controlling the DSV (Digital Sum Value) while conforming to the run-length limiting rule.
At this time, when the predetermined run-length limiting rule is observed, the minimum number of successive 0's occurring between adjacent logic 1's is d (=2) according to the minimum run-length of 3T while the maximum number of successive 0's occurring between adjacent logic 1's is k (=10) according to the maximum run-length of 11T. In other words, when the run-length limiting rule RLL(d, k) (=RLL(2, 10)) is observed, the minimum run-length is set as (d+1)T=3T while the maximum run-length is set as (k+1)T=11T. The restriction of the minimum run-length of (d+1)T=3T and the maximum run-length of (k+1)T=11T is satisfied in the first and second code word sequences 1d and 1f which are obtained by concatenating adjacent code words C and C with an intervening merge bit sequence 1b of three bits. When the first and second code word sequences 1d and if are NRZI-converted, the minimum run-length 3T corresponds to the minimum inversion interval of the NRZI converted sequence while the maximum run-length 11T corresponds to the maximum inversion interval of the NRZI converted sequence.
Then, the EFM signals 1 as p-q modulated (=8–14 modulation) are arranged in order to decrease the DC component and the low frequency components of the EFM signals 1 while conforming to the run-length limiting rule RLL(d, k) (=RLL(2, 10)) of the minimum run-length 3T and the maximum run-length 11T.
As is well known in the art, the NRZI conversion provides a modulation scheme which inverts the signal in a “1” and leaves the signal unchanged for a “0”. Since NRZI (Non Return to Zero Inverted) conversion is performed on the EFM signals 1 including the first and second code word sequences 1d and 1f, the waveform after the NRZI conversion is the waveform of the recording signals R on the glass master disc in which, for example, each L (low) level zone of the recording signals R is represented by a concave pit (or a convex land) while each H (high) level zone of the recording signals R is represented by a convex land (or a concave pit) to provide a bit sequence in combination.
Also, as illustrated in FIGS. 3A and 3B, the above described DSV is the integral of the EFM signals 1 from the start time point of the code word sequence to the current time point in which the H (high) level of the waveform of the EFM signals 1 as NRZI converted is calculated as “1” (positive sense) while the L (low) level thereof is calculated as “−1” (negative sense). At this time, since the polarity is inverted with a data bit of “1” in accordance with the NRZI conversion, a code word generates either of two DSV values dependent upon the state of the waveform obtained by NRZI-converting a code word immediately before the code word to be connected. For example, in the case where the input data word=“002”, the DSV value is inverted when the previous level is the L (low) level shown in FIG. 3A as compared with the DSV value when the previous level is the H (high) level shown in FIG. 3B. Namely, the absolute values of the DSV values as illustrated in FIGS. 3A and 3B are equal to each other in which an input data word (=002) and an input data word (=253) are concatenated with a merge bit sequence.
In this case, a merge bit sequence 1b of three bits is selected from among bit sequences of (000), (001), (010) and (100) in order that the absolute value of the DSV value approaches zero while conforming to the run-length limiting rule RLL(d, k) (=RLL(2, 10)). By such selection, H (high) level zones and L (low) level zones appear in the recording signals R approximately at the same frequency over a long period, while restricting to a low level the DC component of the waveform of the recording signals R. Accordingly, the DSV value is also controlled in order that concave pits and convex lands appear in the disc approximately at the same frequency.
Returning to FIG. 1, one frame of the EFM signals 1 as described above is composed of a synchronization signal 1a, a merge bit sequence 1b, a subcode 1c, a merge bit sequence 1b, a first code word sequence 1d, a merge bit sequence 1b, a C2 error correction code 1e, a merge bit sequence 1b, a second code word sequence 1f, a merge bit sequence 1b, a C1 error correction code 1g and a merge bit sequence 1b arranged in this order from the head, and therefore totals 588 channel bits.
In this case, the synchronization signal 1a arranged in the head is a 24 bit signal including three transitions of 11T, 11T and 2T indicative of the start of a frame and distinctive from the respective signals 1b to 1g as described above.
The subcode 1c arranged after the synchronization signal 1a with the intervening merge bit sequence 1b of three bits is a signal for controlling the playback operation of a CD.
The first code word sequence 1d arranged after the subcode 1c with the intervening merge bit sequence 1b of three bits is prepared by converting each input data word D (each music original data) of p bits (=8 bits) into a corresponding code word C with reference to the coding table as shown in FIG. 2 and concatenating adjacent code words C and C with an intervening merge bit sequence 1b of three bits, and therefore composed of 12 code words C (12 symbols) and 11 merge bit sequences 1b. 
The C2 error correction code 1e arranged after the first code word sequence 1d with the intervening merge bit sequence 1b of three bits is used to perform error correction of the first code word sequence 1d and the second code word sequence if of the EFM signals 1 during the playback of a CD.
Further, the second code word sequence 1f arranged after the C2 error correction code 1e with the intervening merge bit sequence 1b of three bits is composed of 12 code words C (12 symbols) and 11 merge bit sequences 1b of three bits in the same manner as the first code word sequence 1d as described above.
Furthermore, the C1 error correction code 1g arranged after the second code word sequence 1f with the intervening merge bit sequence 1b of three bits is used to perform error correction of the first code word sequence 1d, the second code word sequence 1f and the C2 error correction code 1e of the EFM signals 1 during the playback of a CD.
One block as a unit of music corresponding to a 1/75 second is then composed of successive 98 frames each of which is formed by NRZI-converting one frame of the EFM signals 1 as described above.
Incidentally, the above explanation is applicable to the case of a CD-ROM for storing computer data only by replacing the music original data as shown in FIG. 1 with computer original data, and therefore redundant explanation is not repeated.
Next, a conventional modulation device will be explained with reference to FIG. 4 and FIGS. 5A through 5C.
FIG. 4 is a block diagram schematically showing a conventional modulation device. FIGS. 5A through 5C are views for explaining the calculation of the DSV values for a plurality of the code word sequences temporarily prepared. Each code word sequence is generated in the conventional modulation device by inserting one of the merge bit sequences (000), (001), (010) and (100) between a current code word and the next code word while conforming to a predetermined run-length limiting rule.
The conventional modulation device 20 as shown in FIG. 4 is included in a glass master disc recording apparatus (not shown in the figure) or a CD-R drive which can be used for copying the music information of a CD to a CD-R disc, and composed generally of an 8–14 modulation circuit 21, a merge bit inserting circuit 22, a DSV value calculation circuit 23, a DSV value comparing circuit and merge bit sequence selecting circuit 24.
The conventional modulation device 20 divides music original data AD of 16 bits into upper eight bits and lower eight bits as input data words and then converts the respective input data words D of eight bits into the respective code words C of 14 bits. Then, the conventional modulation device 20 temporarily concatenates, for example, a current code word Cx and a next code word Cy just subsequent thereto with all the intervening merge bit sequences of three bits 1b that satisfy the run-length limiting rule RLL(2, 10) to generate a plurality of code word sequences. Then, the conventional modulation device 20 selects one sequence, whose absolute DSV value is closest to zero, from among the plurality of code word sequences as the final single code word sequence.
More specifically explaining, in the case of the conventional modulation device 20, the music original data AD of 16 bits is input to the 8–14 modulation circuit 21 in chronological order.
In the 8–14 modulation circuit 21 described above, the music original data AD as input is divided into an input data word D of upper eight bits and an input data word D of lower eight bits to generate a series of input data words D of eight bits in chronological order, as explained with reference to FIG. 1, followed by successively converting the respective input data words D of eight bits into code words C of 14 bits with reference to the coding table as shown in FIG. 2, in which for example a current code word Cx and a next code word Cy just subsequent thereto are read in sequence. Then, the current code word Cx and the next code word Cy are input to the merge bit inserting circuit 22 from the 8–14 modulation circuit 21.
Next, the merge bit inserting circuit 22 serves to insert a merge bit sequence of three bits 1b between adjacent code words C and C while conforming to the restriction of the minimum run-length 3T and the maximum run-length 11T in accordance with the run-length limiting rule RLL(2, 10) of the CD standards. Four bit sequences of (000), (001), (010) and (100) are prepared in this merge bit inserting circuit 22 as candidates of the merge bit sequence of three bits 1b. Incidentally, while there are eight sequences of three bits as combinations of “0” and “1”, the remaining four sequences, i.e., (011), (101), (110) and (111) can not satisfy the run-length limiting rule RLL(2, 10) because two or more 1's appear successively or with an intervening “0”, and therefore are not available.
Then, the four merge bit sequences (000), (001), (010) and (100) are temporarily inserted respectively between the code words Cx and Cy as successively input to the merge bit inserting circuit 22 to generate a plurality of code word sequences.
In this case, as illustrated in FIGS. 5A to 5B, for example, the current code word Cx has “010” from the 12th bit to the 14th bit thereof while the next code word Cy is “00100010000010”. On the other hand, the current code word Cx has “1” at the 13th bit position while the next code word Cy has “1” at the third bit position. Thereby, out of the four merge bit sequences as described above, the fourth merge bit sequence (100) does not conform to the run-length limiting rule RLL(2, 10), while the first to third merge bit sequences (000), (001) and (010) conform to the run-length limiting rule RLL(2, 10), and therefore the fourth merge bit sequence (100) is determined not to be inserted.
After inserting the three merge bit sequences (000), (001) and (010) respectively between the code words Cx and Cy, the resultant three code word sequences {Cx(000)Cy}, {Cx(001)Cy} and {Cx(010)Cy} are input to the DSV value calculation circuit 23 to calculate the respective DSV values of the three code word sequences. In the case 1 as shown in FIG. 5A in which the merge bit sequence (000) is inserted between the code words Cx and Cy, the DSV value of the code word sequence {Cx(001)Cy} is +2. Also, in the case 2 as shown in FIG. 5B in which the merge bit sequence (001) is inserted between the code words Cx and Cy, the DSV value of the code word sequence {Cx(001)Cy} is −4. Similarly, in the case 3 as shown in FIG. 5C in which the merge bit sequence (010) is inserted between the code words Cx and Cy, the DSV value of the code word sequence {Cx(010)Cy} is −6.
Thereafter, the respective DSV values of the three code word sequences are input to the DSV value comparing circuit and merge bit selecting circuit 24 from the DSV value calculation circuit 23. The code word sequence {Cx(000)Cy} is then selected by and output from the DSV value comparing circuit and merge bit selecting circuit 24 as a final single code word sequence having the DSV value (=+2) closest to zero from among the three code word sequences. In other words, the DSV value comparing circuit and merge bit selecting circuit 24 serves to select the merge bit sequence (000) corresponding to the final single code word sequence {Cx(000)Cy} having the DSV value closest to zero. The above described procedure is repeated with the next code word Cy as the next current code word and the code word just after the code word Cy as the next next code word to obtain a final single code word sequence {Cx(000)Cy . . . }.
Thereafter, the recording signals R (FIG. 1) suitable for recording is generated from the final single code word sequence {Cx(000)Cy . . . } of which the DSV value is controlled. Then, the generated recording signals R is recorded on a glass master disc for CD or a CD-R by a laser beam.
The glass master disc for CD is used to prepare a stamper disc (not shown in the figure) which in turn is used to produce a CD.
In view of the above, utilizing a software for copy stored in a hard disk (not shown) in a personal computer (not shown), a user can play back a CD on which music information that is desired to be copied by the user is recorded, with a CD drive (not shown) and then input the music information that is outputted from the CD drive and is desired to be copied on a CD-R, into a CD-R drive (not shown), and accordingly can copy the music information desired to be copied, onto the CD-R as it is by means of the conventional modulation apparatus 20 provided in the CD-R drive without authorization of the owner of the copyright.
In other words, when the music original data AD of 16 bits as output from the CD drive is encoded by the conventional modulation device 20 in the CD-R drive, the music information recorded on the CD-R is identical to the music information recorded on the CD in terms of the EFM signals. The CD-R as duplicated can be used to copy the music information onto another CD-R again without the original CD so that the music information is distributed in large quantities in the world.
Taking into consideration the situation, an exemplary copy-protected optical disc is proposed which can be used to prevent the music information recorded on a CD or the computer data recorded on a CD-ROM from being copied onto a recordable CD-R or CD-RW (for example, disclosed in Japanese Patent Application Laid-open No.2001-357536, pages 4–5, FIG. 4).
FIG. 6 is a longitudinal cross sectional view showing an optical disc, in which copy-protection technology is incorporated, as an exemplary conventional technique.
The conventional optical disc 100 as shown in FIG. 6 is one described in the above Japanese Patent Application Laid-open No. 2001-357536. Referring to the same document, in the conventional optical disc 100 designed by incorporating the copy-protection technology in an optical disc such as CD-ROM, DVD-ROM, convex and concave portions are formed with lengths ranging from 3T to 14T (T is 0.133 μm) based on the run-length limiting rule (referred to as the run-length limited encoding scheme in the same publication). However, this technique is characterized in that the sequence of the convex and concave portions further includes convex or concave portions having each length shorter than the minimum run-length based on the run-length limiting rule.
Specifically, as shown in FIG. 6, a pit A is convexly formed with a length of 1T to 2T while a pit B is concavely formed with a length of 1T to 2T located apart from the pit A by X. The lengths of these pits A and B do not satisfy the run-length limiting rule.
Accordingly, the shortest pit length (or the shortest land length) of the conventional optical disc 100 is smaller than the normal value by setting a minimum run-length of 1T to 2T without satisfying the requirement of the minimum run-length 3T out of the run-length limiting rule comprising the minimum run-length 3T and the maximum run-length 14T.
In order to implement the technical concept of the conventional optical disc 100 as described above within a known CD on which convex and concave portions (pit sequence) are formed with lengths between 3T to 11T conforming to the run-length limiting rule RLL (2, 10) included in the CD standards to modify the signal sequence recorded on a CD, a code word C such as “00100110010010” is used as one corresponding an input data word D (=255) to introduce a short convex or concave portion (pit sequence) of 1T instead of “00100000010010” in the coding table shown in FIG. 2.
When playing the modified CD with a commercially available optical disc drive, it is impossible to judge whether or not the optical disc as played is legally-distributed because the verification pit having a length of 1T to 2T in the data as read out is shorter than a normal pit having a pit length (land length) of 3T to 11T and therefore the RF signal as read out by an optical pickup does not reach a sufficient bright level or a sufficient dark level so that the binary signals obtained from the RF signal contains no signal indicative of the verification pit having a length of 1T to 2T. Also, it is possible without trouble to copy the music information recorded on the modified CD by reproducing the music information with a commercially available optical disc drive and inputting the reproduced music information to a CD-R drive.
Accordingly, under the present circumstance in which a number of players and CD-R drives have already been distributed on the market, the above copy-protection mechanism is not effective because of its assumption of the spread of a new player such as the optical disc 100 capable of detecting the verification pit.
Furthermore, in the same manner as optical discs, digital magnetic tapes have the same problem that digital information signals recorded therein are copied.